Re: What is the difference between UNIQUE and PRIMARY KEY?

From: Brian Tkatch <SPAMBLOCK.Maxwell_Smart_at_ThePentagon.com.SPAMBLOCK>
Date: Wed, 11 Apr 2001 22:26:13 GMT
Message-ID: <3ad4d887.91294906@news.alt.net>

On Wed, 11 Apr 2001 22:17:29 +0100, "Bart" <Bart_at_opcomp.demon.co.uk> wrote:

>A primary key is, by definition, the primary key to a table and a table can
>have only one.
>
>A primary key is not an index, although is usually has an associated index.
Usually? It always has one. If you do not specify one in the clause, it will create one of the same name.
>
>A table can have any number of unique indexes.
>
>A foreign key can only reference a primary key.
You mean a Foreign Key when not specifiying a column looks for the primary key. It can reference either a primary or a unique key.

create table a (a number unique, b number references a(a))

I would guess a Primary Key is the same thing as not null and unique index. It just is the default reference for foreign keys, and other programs (such as access) which require a key for updating, will look for the primary key.

Brian Received on Wed Apr 11 2001 - 17:26:13 CDT